Prosthetic heart valves are used to replace damaged or diseased heart valves. In vertebrate animals, the heart is a hollow muscular organ having four pumping chambers: the left and right atria and the left and right ventricles, each provided with its own one-way valve. The natural heart valves are identified as the aortic, mitral (or bicuspid), tricuspid, and pulmonary valves. Prosthetic heart valves can be used to replace any of these natural valves. The two primary types of prosthetic heart valves known in the art are mechanical valves and bio- prosthetic valves. Mechanical valves include rigid leaflets and a pivoting mechanism, and bio- prosthetic valves utilize flexible tissue leaflets, typically mounted to a manufactured support frame. The present invention provides methods for selecting leaflets in bio-prosthetic valves.
Bio-prosthetic valves may be formed from an intact, multi-leaflet porcine (pig) heart valve, or by shaping a plurality of individual leaflets out of bovine pericardial tissue and combining the leaflets to form the valve. The pericardium is a sac around the heart of vertebrate animals, and bovine (cow) pericardium is commonly used to make individual leaflets for prosthetic heart valves. The bovine pericardium is first harvested from the animal and then chemically fixed to crosslink collagen and elastin molecules in the tissue and increase the tissue durability, before being cut into leaflets. Various physical characteristics of the tissue may be examined before or after fixation.
One drawback faced by a patient having an implanted bio-prosthetic heart valve is the potential for calcification of the leaflets if the valve remains in place for an extended period of time (more than ten years). Calcification tends to make the leaflets less flexible. A significant amount of research has been accomplished in mitigating calcification of bovine pericardial leaflets to lengthen the useable life of the heart valve. Calcification may reduce the performance of the heart valve, and thus, the highest quality materials and design in the heart valve is required to forestall a failure of the valve from excessive calcium deposits.
Despite the drawbacks of artificial heart valve material, over twenty years of clinical experience surrounding implanted artificial heart valves has produced a proven track record of success. Research in extending the useful life of the bio-prosthetic valves continues, however. One aspect of designing heart valves which is very important in improving their performance is the selection of the pericardial tissue used in the leaflets. In all heart valves, the natural action of the flexible heart valve leaflets, which seal against each other, or co-apt, is desirable. The difficulty in simulating the leaflet movement of an actual heart valve (especially a mitral valve) in a prosthetic valve is that the leaflets used are "inanimate." There are no muscular attachments to the leaflets as in the natural valve, and the prosthetic leaflets must co-apt to function properly solely in response to the fluid pressures within the heart chambers. Indeed, natural coaptation of the leaflets in bio-prosthetic valves comprising a plurality of individual leaflets sewn together is particularly difficult, even when compared to inanimate but intact valves, such as harvested porcine valves.
Much of this research involves the mechanical properties of fresh or fixed bovine pericardium. A good discussion of the various physical properties of fixed bovine pericardium is given in Simonescu, et al, Mapping of Glutaraldehyde-Treated Bovine Pericardium and Tissue Selection For Bio-prosthetic Heart Valves, Journal of Bio-Medical Materials Research, Vol. 27, 1993. Simionescu, et al, recognized the sometimes striking variations in physical properties of the pericardial tissue, even in the same pericardial sac. Their research mapped out areas in individual pericardial sacs and tested those areas for fiber orientation, suture holding power, and thickness. In another paper by Sacks, Bi-axial Mechanical Behavior of Fixed Bovine Pericardium, Fifth World Biomaterials Congress, May-June 1996, the collagen fiber architecture within bovine. pericardial tissue was examined and various specimens were tested in a bi-axial tester. The results indicated that by presorting for uniform collagen fiber architecture, more uniform bio- pericardial specimens could be obtained for better controlled use in bioprosthetic applications. Finally, in another study, Zioupos, et al, Anisotropic Elasticity and Strength of Glutaraldehyde Fixed Bovine Pericardium For Use In Pericardial Bioprosthetic Valves, Journal of Biomedical Materials Research, Vol. 28, 1994, various tests were performed on fixed bovine pericardial tissue to determine the stress/strain behavior along various axes. The results suggest that leaflets can be made from fixed bovine pericardium possessing pronounced anisotropy in strength and stiffness along two orthogonal directions. In the leaflets circumferential direction, which bears most of the stress during function, the stiffer pericardium is desired, while in the radial direction, more flexible tissue is desired. Leaflets are thus cut from bulk tissue whose properties have generally been examined, and the leaflets categorized accordingly. Despite the extensive research into bulk tissue characteristics there remains a need for a more reliable method of selecting leaflets to insure maximum functional compatibility with the other leaflets in the dynamic operating environment of a prosthetic heart valve.